1. Field of the Invention
The present invention relates generally to a handover method for minimizing a packet call reconnection delay time between different mobile communication schemes when a packet call is disconnected due to a handover between communication networks providing a communication service using different mobile communication technologies and a multi-mode terminal for the same.
2. Description of the Related Art
Recently, mobile communication networks are being upgraded from second generation (2G) networks, such as code division multiple access (CDMA) networks and European global systems for mobile communication (GSM) networks, to third generation (3G) networks such as wideband CDMA (WCDMA) networks. Since service areas using 2G schemes are widely distributed and their infrastructures are completely equipped at the present time, while the infrastructures of the 2G networks are transitionally used, areas in which the 3G networks are installed are gradually widening. Thus, in this transitional state, terminals designed to support services specified to the 3G networks are manufactured as multi-mode terminals to also support the existing 2G schemes.
Some main characteristics of such a multi-mode terminal are an idle handover function for changing to a CDMA mode or a WCDMA mode, a traffic handover function for seamlessly changing to the CDMA mode when the multi-mode terminal moves to a WCDMA shadow area while engaged in communication, and a packet call reconnection function for reconnecting a packet type data call by changing to the CDMA mode or the WCDMA mode when a handover occurs. Although these functions are economical because an existing 2G CDMA network can be used, since the multi-mode terminal must quickly perform a handover between different modems, hardware and software implementation of the multi-mode terminal is complicated.
FIG. 1 is a signaling diagram illustrating an operation of a multi-mode terminal performing the packet call reconnection function.
Referring to FIG. 1, in response to a packet service connection request from a user, an application processor, i.e., a controller 30 transmits a Packet-Switched (PS) CALL ORIGINATION message to a WCDMA modem 20 in step 100. In response to the PS CALL ORIGINATION message, the WCDMA modem 20 sequentially transmits a GPRS (General Packet Radio Service) Mobility Management (GMM) SERVICE REQUEST message in step 105 and an ACTIVATE Packet Data Protocol (PDP) CONTEXT REQUEST message in step 110 to a WCDMA Base Transceiver Station (BTS) 10 and receives an ACTIVATE PDP CONTEXT RESPONSE message from the WCDMA BTS 10 in step 115. In this case, the service request is achieved through protocol layer GMM. Then, the WCDMA modem 20 recognizes that a packet call is connected through the ACTIVATE PDP CONTEXT RESPONSE message and transmits a CALL CONNECT message to the controller 30 in step 120. Accordingly, a packet type path (packet bearer) is established in step 130.
However, if the multi-mode terminal moves to a shadow area or a boundary area in the middle of packet data transmission, the multi-mode terminal may not communicate with the WCDMA BTS 10, thereby occurring a radio link failure state in step 140. Then, to reconnect a packet call from a WCDMA network to a CDMA network, the multi-mode terminal must disconnect the packet call from the WCDMA network, change to a handover target mode, and reconnect a packet call to the CDMA network. Accordingly, the WCDMA modem 20 activates a timer, and if the timer expires, the WCDMA modem 20 transmits a RADIO LINK FAILURE INDICATION message to the controller 30 in step 145. Then, a mode change, such as a switching process, is performed to change to the CDMA mode under the control of the controller 30 in step 150. Then, a CDMA modem 40 establishes a packet type path (packet bearer) with a CDMA BTS 50, which is a handover target network, in step 155 by connecting to the CDMA BTS 50. As described above, a delay because of the reconnection occurs before the packet type path (packet bearer) with the handover target network is established after the radio link failure. It is important in the packet call reconnection function to reduce the delay time of the reconnection process.
FIG. 2 is a flowchart illustrating a process of disconnecting a radio link among the process described in FIG. 1.
Referring to FIG. 2, in a packet call connection state, if a radio link between the WCDMA BTS 10 and the WCDMA modem 20 is disconnected in step 200, a physical layer 22 of the WCDMA modem 20 transmits an OUT-OF-SYNC message to a radio source control layer 24, which is an upper layer of the physical layer 22, in step 205. The radio source control layer 24 determines in step 210 whether the number of received OUT-OF-SYNC messages is N313, and if N313 OUT-OF-SYNC messages are consecutively received, the radio source control layer 24 activates a T313 timer in step 215. The physical layer 22 consecutively transmits an IN-SYNC message to the radio source control layer 24 in step 220, and the radio source control layer 24 determines in step 225 whether the T313 timer expires. If N315 IN-SYNC messages have not been received when the T313 timer expires, the radio source control layer 24 generates a RADIO LINK FAILURE event in step 230 and disconnects a relevant packet call. Thus, the multi-mode terminal performs nothing during a NO SERVICE period until the multi-mode terminal generates the RADIO LINK FAILURE event based on N313, T313, and N315 in a shadow area. Herein, since N313, T313, and N315, which are reference values used to define RADIO LINK FAILURE in WCDMA, are included in a System Information Base (SIB) 1 provided by a WCDMA BTS when the multi-mode terminal in the WCDMA mode registers in the WCDMA BTS and are defined in the standard, a detailed description is omitted.
As described above, if a radio link is disconnected, only after a packet call is successfully connected to a handover target network, can the user use a packet service again. That is, a delay time for which the user cannot use the packet service occurs from when communication with a WCDMA BTS in the WCDMA mode is disconnected to when a packet call is reconnected in the CDMA mode. The delay time is around 20˜40 seconds including a network state, parameters defined in SIB1, and a switching time in a mode change. Although the packet call reconnection function has an advantage that the user does not have to directly perform a reconnection process, since the user cannot use a service for a very long time, the multi-mode terminal may seem to be in a malfunctioning state.
As described above, conventionally, it is important to perform a handover without disconnecting a packet call when the handover is required in a multi-mode terminal communicating with networks using different mobile communication technologies, e.g., a CDMA network and a WCDMA network. Accordingly, when a packet call is disconnected due to a handover in a conventional dual-mode terminal, an application in use can no longer provide a service. Thus, to continuously use a previous service, a packet call reconnection is performed. However, a delay time according to the packet call reconnection may be as long as around 20˜40 seconds, and a user must wait with a fixed screen image until a packet call is reconnected, thereby causing great inconvenience to the user.